This invention relates generally to long stroke, well pumping units and more specifically to an uncomplicated velocity control and improved drive for imparting reciprocating movement to the polish rod of the pump. The invention includes an improved and reliable reciprocating drive wherein a dwell period is provided between an upstroke and a downstroke during which the power source is in an off position, and a velocity control which senses the velocity of the polish rod during the aforementioned dwell period and energizes the power source when a predetermined velocity is achieved following reversal of direction. Generally, the predetermined velocity is selected to correspond with the normal operating speed of the system and power is applied or dissipated by the power source when the predetermined velocity is achieved to prevent under speed or over speed of the system, respectively. In this manner, the velocity control automatically adjusts the system to accommodate varying load conditions due to changes in the gas/oil/water ratios, known as "buoyancy" of the well, and friction within the system. This adaptability results in substantially increased efficiency and conservation of energy, while at the same time reducing wear and tear on the system and increasing the life of the unit.
The invention has particular utility with a long stroke, well pump employing a reversible motor as the power source. Such a well pumping unit, as herein disclosed, includes a tower mounted on a base platform, a source of power in the form of an electric or hydraulic motor, a winding drum on the base platform driven from the electric or hydraulic motor, a lift belt made of conveyor belting extending from the winding drum up the tower and over a spool mounted atop thereof and then downwardly to the polish rod of the otherwise conventional well pump. A counterweight or weight box is interposed in that portion of the lift belt between the spool and the winding drum so that power requirements are kept to a minimum. An idler spool is provided at the base of the tower and that portion of the lift belt between the counterweight and the winding drum is trained beneath the idler spool so as to eliminate any side to side movement of the counterweight during operation of the pump. Mechanism is designed and arranged to interrupt the power to the motor for a predetermined dwell period as the polish rod approaches completion of an upstroke, and to reverse the direction of the motor after the polish rod decelerates, reverses direction and travels a predetermined distance in the downstroke. The reversing mechanism is provided with an adjustment feature which allows the time of interruption and the duration of the dwell period to be adjusted, thereby allowing the length of stroke to be readily changed, without shutting down the unit. The velocity control contemplated by the invention is designed and arranged to be responsive to the velocity of the polish rod, sense the direction of rotation of the motor and energize the motor in the desired direction of rotation when the polish rod achieves a predetermined velocity. The predetermined velocity is normally set at the synchronous speed of the motor, but may be set at other speeds, if desired, depending upon the specific starting requirements. Since an induction motor's synchronous speed will vary slightly with load, we will define synchronous speed to include the entire normal operating range of the motor in a given system.
A brief description of the background and development of well pumping units is appropriate. In the early life of a well, reservoir pressure alone may be sufficient to lift the oil to the surface, providing local regulatory authorities permit such a procedure. However, such pressure is eventually exhausted whereupon the oil must be pumped to the surface. The most common variety of pump in use is a walking beam pump having a nominal stroke of approximately seven to twelve feet. A walking beam pump is suitable for shallow to medium depth wells, but such a pump becomes increasingly inefficient as well depth and stroke frequency increases. Specifically, rod stretch, dynamics and pump volumetric efficiency combine to decrease efficiency as well depth and stroke frequency increases.
Thus, long stroke well pumping units, particularly useful in deep wells, have been developed, some having stroke lengths of thirty-two feet or more. An early example of such a prior art long stroke pumping unit is the long stroke pumping mechanism disclosed and claimed in prior U.S. Pat. No. 1,970,596 issued to Clarence J. Coberly. Coberly's mechanism includes a cable system connected at one end to the polish rod and extending up and over a sheave mounted on a tower above the well, down and around a drum mounted at the base of the tower, and connected at its other end to a counterweight arrangement. The drum is driven in first one direction and then the other by a reversible motor to provide reciprocating motion to the polish rod, and a system of cams on a shaft are positively driven by rotation of the drum through appropriate gear reduction to actuate sequentially a series of spring contracts controlling the direction and magnitude of current to the motor to accelerate the motor harmonically from zero to maximum speed, maintain it and deccelerate to zero again, reverse current direction to the motor and then repeat the operation, thereby to rotate the drum in alternate directions and reciprocate the polish rod. It is noteworthy that reciprocation is achieved by motor reversal only, and that acceleration and decceleration is achieved in response to cam sequence and rotation corresponding to fixed increments of travel (distance) regardless of speed.
An improved prior art long stroke pumping unit is the wire line deep well pumping apparatus disclosed and claimed in prior U.S. Pat. No. 3,248,958 issued to Emil A. Bender. The long stroke pumping unit discussed therein is a basic yo-yo variety which has a power system in which a cycle of takeup (during pump upstroke) and payout (during pump downstroke) is accomplished without the need for winding drum reversal; thus, the power source of the unit is reversed only after a full cycle of operation rather than with each stroke, as in the case of the Coberly unit. As disclosed in the Bender Patent, a simple limit switch (responsive only to distance of travel) is disclosed for reversing the electric motor and although the Patent further suggests that polish rod stroke and time delay (which at best introduces a fixed time response) may be modulated, no structure or system is disclosed for accomplishing such results. Another prior U.S. Pat. No. 3,345,950 to Emil A. Bender discloses a long stroke, well pumping unit either electrically or hydraulically powered and including a limit switch system alternately operated by the yoke suspending the polish rod and the counterweight (fixed distance response) to effect power source reversal.
Yet another invention to Emil A. Bender, U.S. Pat. No. 4,391,155, involves a well pumping unit wherein a yo-yo drive as above discussed is employed with a flexible lift belt being the operative connection between the winding drum and the polish rod, and power source reversal is positively associated with the winding drum rather than other components of the system. In this unit, a reversing mechanism is provided including a chain and sprocket transmission operable from the winding drum and rotating a contact for a limit switch. The reversing control mechanism consists of two side by side disks configured with identical chordal edges over a significant portion of the circumference to allow manual, relative adjustment to define a V-shaped notch in the combined circumferential camming surface. The disks cooperate with a 3-position switch so that the switch is spring-biased to a neutral (power-off) position when disposed in the V-shape notch, and is cammed to a second (power on in one direction) position upon rotation of the disks in one direction and cammed to a third (power on in the opposite direction) position upon reverse rotation of the disks. Here again, a fixed distance (albeit adjustable) reversing control is taught. Furthermore, in order to adjust the reversing mechanism, the operation of the unit must be terminated and the reversing mechanism partially disassembled, and precise centering of the 3-position switch and exact adjustment of the disks to define co-equal spaces either side of the center-line of the V-shaped notch, is essential in order to establish and maintain a uniform length of stroke in either direction of rotation of the winding drum.
An improved reciprocating drive and control mechanism invented by Tam Duc Le and Weems D. Turner is the subject of a co-pending application, Ser. No. 489,826, filed Apr. 29, 1983 and assigned to the assignee of the present invention. In this example of the prior art, a reversing mechanism is responsive to rotation of a winding drum and includes a cam in the form of an eccentric disk and a 3-position switch arranged to reverse the direction of the power source when a predetermined stroke (responsive to a fixed distance) has been completed. The relative positions of the cam and the 3-position switch may be manually adjusted while the unit is operating to modify the length of stroke to the desired dimension.
Other prior art reversing or control mechanisms in similar pump applications all include a simple limit switch (fixed distance response) similar to the examples described above. Such prior art neither teaches, nor recognizes the substantial advantages of, a unique control mechanism responsive to the velocity of the system and designed to energize the power source in the proper direction when the velocity of the system reaches a predetermined operating speed.